feat(updater): in-app update on desktop, releases link on Android

Anyone who installed an AppImage or ran the Windows installer was frozen
on that version forever. Nothing in the app ever mentioned a new release
existed, and the release notes were the only announcement.

Desktop now checks a signed manifest, shows the version and its notes in
Settings, and installs and relaunches on request. The signature check is
the whole point: it is what stops a substituted download from being
installed by the app itself. Windows binaries stay unsigned for
SmartScreen purposes -- that is a code-signing certificate, a separate
problem -- but the update payload is verified against our own key.

Android is deliberately not wired to the updater. An app may not replace
its own APK; that is the package installer's job, and the plugin has no
Android implementation. It gets a link to the releases page instead of a
button that would throw.

The plugins are gated with a target-triple cfg rather than
cfg(desktop). Cargo only evaluates target cfgs in a [target.'cfg(..)']
table, so cfg(desktop) matches nothing, silently drops the dependency,
and fails much later with "Permission updater:default not found" -- which
is exactly what the first attempt here did.

Where the manifest lives took some finding. This Gitea serves
/releases/download/<tag>/<asset> but 404s on
/releases/latest/download/<asset> (verified against a real asset), so
there is no stable latest-release URL. The gitea-pages branch is
force-pushed wholesale by publish-docs.yml, so it cannot host the file
either. latest.json therefore gets its own orphan branch, read over the
raw-file URL, and is published from a scratch repo in RUNNER_TEMP rather
than by switching branches in the checkout -- doing that would have left
the following steps standing on a one-commit history, and the next step
but one runs release:notes against the real commit range.

Also fixed, all of it release-integrity:

  - "appimage" is in bundle.targets. The release notes have advertised an
    AppImage for months; tauri.conf.json never built one, the artifact
    step globbed for *.AppImage, found nothing, and said nothing. The
    step now fails instead.
  - The .AppImage.tar.gz/.sig pair and the NSIS .sig are collected. A
    manifest referencing a signature that was never uploaded fails only
    on the user's machine, so the manifest step also refuses to write an
    entry with an empty signature.
  - Release notes are generated by release:notes from the traceability
    graph, which is what CLAUDE.md has asked for all along, instead of a
    fixed heredoc that said "see CHANGELOG.md for detailed changes" and
    linked "GitHub Issues" on a Gitea-hosted project.
  - The notes tell users how to verify a download with SHA256SUMS.

Requirements UR-077 / DR-217, tests UT-208 (12 cases over the version
comparison and the platform decision, including that a pre-release does
not offer itself as an upgrade to the matching release).

Verified: 1070 frontend tests, cargo check for both the host and
aarch64-linux-android (confirming the plugins are absent there), clippy
-D warnings, svelte-check 0 errors.

bun.lock

@@ -8,6 +8,8 @@
         "@tauri-apps/api": "^2",
         "@tauri-apps/plugin-opener": "^2",
         "@tauri-apps/plugin-os": "^2.3.2",
+        "@tauri-apps/plugin-process": "^2.3.1",
+        "@tauri-apps/plugin-updater": "^2.10.1",
         "hls.js": "^1.6.15",
         "svelte-dnd-action": "^0.9.69",
       },
@@ -282,6 +284,10 @@
 
     "@tauri-apps/plugin-os": ["@tauri-apps/plugin-os@2.3.2", "", { "dependencies": { "@tauri-apps/api": "^2.8.0" } }, "sha512-n+nXWeuSeF9wcEsSPmRnBEGrRgOy6jjkSU+UVCOV8YUGKb2erhDOxis7IqRXiRVHhY8XMKks00BJ0OAdkpf6+A=="],
 
+    "@tauri-apps/plugin-process": ["@tauri-apps/plugin-process@2.3.1", "", { "dependencies": { "@tauri-apps/api": "^2.8.0" } }, "sha512-nCa4fGVaDL/B9ai03VyPOjfAHRHSBz5v6F/ObsB73r/dA3MHHhZtldaDMIc0V/pnUw9ehzr2iEG+XkSEyC0JJA=="],
+
+    "@tauri-apps/plugin-updater": ["@tauri-apps/plugin-updater@2.10.1", "", { "dependencies": { "@tauri-apps/api": "^2.10.1" } }, "sha512-NFYMg+tWOZPJdzE/PpFj2qfqwAWwNS3kXrb1tm1gnBJ9mYzZ4WDRrwy8udzWoAnfGCHLuePNLY1WVCNHnh3eRA=="],
+
     "@testing-library/dom": ["@testing-library/dom@10.4.1", "", { "dependencies": { "@babel/code-frame": "^7.10.4", "@babel/runtime": "^7.12.5", "@types/aria-query": "^5.0.1", "aria-query": "5.3.0", "dom-accessibility-api": "^0.5.9", "lz-string": "^1.5.0", "picocolors": "1.1.1", "pretty-format": "^27.0.2" } }, "sha512-o4PXJQidqJl82ckFaXUeoAW+XysPLauYI43Abki5hABd853iMhitooc6znOnczgbTYmEP6U6/y1ZyKAIsvMKGg=="],
 
     "@testing-library/svelte": ["@testing-library/svelte@5.3.1", "", { "dependencies": { "@testing-library/dom": "9.x.x || 10.x.x", "@testing-library/svelte-core": "1.0.0" }, "peerDependencies": { "svelte": "^3 || ^4 || ^5 || ^5.0.0-next.0", "vite": "*", "vitest": "*" }, "optionalPeers": ["vite", "vitest"] }, "sha512-8Ez7ZOqW5geRf9PF5rkuopODe5RGy3I9XR+kc7zHh26gBiktLaxTfKmhlGaSHYUOTQE7wFsLMN9xCJVCszw47w=="],
@@ -752,6 +758,8 @@
 
     "@tailwindcss/oxide-wasm32-wasi/tslib": ["tslib@2.8.1", "", { "bundled": true }, "sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w=="],
 
+    "@tauri-apps/plugin-updater/@tauri-apps/api": ["@tauri-apps/api@2.11.1", "", {}, "sha512-M2FPuYND2m+wh5hfW9ZpSdxMPdEJovPBWwoHJmwUpysTYNHaOkVFN419m/K0LIgjb/7KU2vBgsUepJWugQCvAA=="],
+
     "@testing-library/dom/aria-query": ["aria-query@5.3.0", "", { "dependencies": { "dequal": "^2.0.3" } }, "sha512-b0P0sZPKtyu8HkeRAfCq0IfURZK+SuwMjY1UXGBU27wpAiTwQAIlq56IbIO+ytk/JjS1fMR14ee5WBBfKi5J6A=="],
 
     "@typescript-eslint/eslint-plugin/ignore": ["ignore@7.0.6", "", {}, "sha512-BAg6QkE8W+TuQLrrw0Ugr7HegXduRuuj8/ti2kSOc+jz1dmx8/WNcjr6XGnq5YpDWxFwwaavqD0+jIUOKelTsw=="],