docs: specs, requirements, ux-flows and traceability for new features

Add specs for the account menu, downloads-as-offline-library, offline
downloaded-only filter, and scoped search (+ boundary revision). Add the
new UR/DR entries to requirements.md, update ux-flows, and regenerate the
traceability matrix.

TRACES: UR-049, UR-050, UR-052, UR-053, UR-054, UR-055, UR-056

docs/specs/SPEC-TEMPLATE.md

@@ -0,0 +1,105 @@
+# Spec: <feature name>
+
+<!--
+  Copy this file to docs/specs/<kebab-name>.md and fill it in. Delete the HTML
+  comments as you go. The section that matters most for this project is
+  "Layer assignment" — read its comment before writing it.
+
+  Before merging a spec, run it past docs/specs/SPEC-REVIEW-CHECKLIST.md.
+-->
+
+**Status:** Proposed <!-- Proposed | Accepted | Implemented | Superseded -->
+**Requirements:** <!-- UR-xxx → DR-yyy; allocate new DRs in requirements.md. -->
+**UX spec:** <!-- link to the relevant ux-flows.md section, or "n/a". -->
+**Supersedes / revises:** <!-- link any spec this changes, or delete this line. -->
+
+## Summary
+
+<!-- 2–4 sentences. What changes for the user, in plain terms. -->
+
+## Motivation
+
+<!-- Why now. The problem being solved. -->
+
+## Layer assignment
+
+<!--
+  🔴 THIS IS THE SECTION THAT KEEPS THE ARCHITECTURE HONEST. Do not skip it, and
+  do NOT reframe it as "how little backend work can we get away with."
+
+  The project rule (CLAUDE.md, architecture/02-svelte-frontend.md): the Rust
+  backend owns ALL business logic — auth, catalog, sessions, downloads, offline,
+  playback, AND domain vocabulary (e.g. what Jellyfin item types the category
+  "Music" means). The Svelte frontend is PRESENTATION ONLY: rendering, layout,
+  navigation, view/order preferences, input handling.
+
+  For each distinct piece of *logic* this feature introduces, put it in the table
+  and name the layer it belongs to and WHY. "It's less work in the frontend" and
+  "the backend already accepts this parameter" are NOT reasons to place logic in
+  the frontend — the backend accepting a parameter does not make deciding that
+  parameter's value a presentation concern.
+
+  Litmus test for "does this belong in Rust?": Would this logic have to change if
+  Jellyfin changed its API, added an item type, or altered a business rule? If
+  yes, it is domain logic → Rust. Would it change if we redesigned the UI? If
+  yes (and only yes), it is presentation → frontend.
+
+  A past incident: scoped-search.md placed the item-type taxonomy (what "Music"
+  means as a set of Jellyfin types) in the frontend because the backend already
+  accepted an includeItemTypes filter. That was a boundary leak; see
+  scoped-search-boundary.md. This section exists to catch that class of mistake
+  at spec time, not in review three features later.
+-->
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| <!-- e.g. scope → item-types --> | Rust | <!-- domain vocabulary; changes with Jellyfin's API --> |
+| <!-- e.g. group display order --> | Frontend | <!-- pure presentation; changes only if UI is redesigned --> |
+
+<!--
+  If a row is genuinely borderline, say so and give the tie-breaker you used.
+  Borderline defaults to Rust for anything touching domain data or vocabulary.
+-->
+
+## Design
+
+<!--
+  How it works. Wire shapes for anything crossing the IPC boundary. Remember:
+  - Command NAME must match the Rust fn name exactly.
+  - Top-level params auto-convert snake_case → camelCase (Tauri v2).
+  - Nested struct fields need #[serde(rename_all = "camelCase")].
+  - Events are kebab-case.
+  (See CLAUDE.md §IPC and architecture/04-type-sync-and-threading.md.)
+
+  Regenerate bindings.ts from Rust types; never hand-edit it.
+-->
+
+## Out of scope
+
+<!-- What this spec deliberately does NOT do. -->
+
+## Acceptance criteria
+
+<!-- Checkable statements. Include the standard gates: -->
+
+- [ ] `bun run check` and `bun run test` pass.
+- [ ] `cargo fmt` clean, `cargo clippy` clean, `bun run test:rust` passes (if Rust changed).
+- [ ] `bun run check:boundary` passes (no taxonomy leak into the frontend).
+- [ ] New requirement-implementing code carries `// TRACES:` comments.
+- [ ] `bindings.ts` regenerated if Rust types changed.
+
+## Testing
+
+<!-- Rust: cargo test. Frontend: vitest, src/lib/**/*.test.ts. What to cover. -->
+
+## TRACES
+
+<!-- Suggested tags per new/changed piece: UR-xxx | DR-yyy | tests. -->
+
+## Notes for the implementer
+
+<!--
+  - A parallel Claude session may be active in this repo — `git diff` before
+    "repairing" unexpected changes (see project memory / CLAUDE.md gotchas).
+  - Anything else non-obvious.
+-->