feat(search): add node label search and source filtering

[w-winter]

Pi session files support append-only node labels, but pi-session-manager was dropping that data before it reached the search index and wasn't resolving it reliably in the session tree. As a result, labeled nodes couldn't be searched globally, content_only vs labels_only behavior didn't exist, and large chunked sessions could show incomplete label state locally.

This change makes labels first-class searchable node metadata in the two UIs I believe to be most relevant: global full-text search and search of the in-session tree. Pi-specific label semantics are mirrored with a raw session parser that resolves labels by whole-file order with latest-wins and empty-label-clears behavior. Those resolved labels are indexed as source_type = "label" rows tied to the target node, ranked ahead of ordinary content hits for the same node, and exposed through one canonical sourceFilter state: all, labels_only, or content_only.

• On the global search side, the mounted CommandMenu path now supports source filtering, leading #all / #labels / #content tokens, and labels_only + empty query browse-all-labels mode.
• On the session viewer side, resolved labels are fetched at the sidebar boundary via getRuntimeSessionLabels(sessionPath), so tree display/search/filter behavior works correctly even when the visible entries are only a chunk of the full file.

Summary of changes:

• Pi label parsing and indexing: add src-tauri/src/domain/pi_session.rs, preserve target_id / label on session entries, and emit label-backed message_entries rows from src-tauri/src/data/sqlite/message_index.rs
• Global search semantics: extend src-tauri/src/commands/search.rs with source_filter, label-hit precedence, match_reason = "label", and dedicated labels_only + empty query browse mode
• Resolved label lookup for the tree: add cached get_session_labels support in src-tauri/src/commands/session_file.rs / session.rs, then wire it through runtime providers and src/runtime-data/sessionSource.ts
• Mounted search UI: update src/components/command/CommandMenu.tsx / CommandPalette.tsx so the canonical global-search surface supports the shared source-filter state, leading # filter tokens, label badges, and browse-all-labels behavior without rewriting the query text when the selector changes
• Session tree behavior: make src/utils/session.ts, src/utils/session-tree.ts, src/components/session-tree/SessionTree.tsx, and src/hooks/useSessionTreeLookup.ts label-aware so labeled nodes display correctly, participate in local search, respect the labeled-only filter, preserve tree structure for Pi-specific entry types, and navigate via target nodes rather than raw label mutations
• Sidebar lifecycle: update src/components/session-viewer/SessionViewerSidebar.tsx to fetch labels only while visible, key label state by sessionPath, clear current-session labels on fetch failure, and avoid reusing previous-session labels during session switches
• Runtime parity: align src/demo/search.ts and src/browser-dataset/search.ts with backend source-filter and dedupe semantics so non-backend runtimes behave consistently
• Locales and shared helpers: add source-filter / label-match strings across search locales and add shared source-filter token helpers in src/utils/search.ts
• Tests and tooling: add and expand Rust and frontend coverage for label parsing, search filtering/browse behavior, label precedence, excluded-session handling, tree label resolution, sidebar lifecycle, and runtime parity, and declare/sync the frontend test-tooling dependencies needed for the added Vitest/jsdom coverage in package.json and pnpm-lock.yaml
• Docs: update README.md highlights and CHANGELOG.md unreleased notes

Checks:

• pnpm install --frozen-lockfile passes
• cargo fmt --all --check passes
• cd src-tauri && cargo clippy -- -D warnings passes
• cd src-tauri && cargo test is still not green due to pre-existing unrelated failures:
  • all platforms: commands::settings::tests::save_external_session_providers_core_clears_disabled_provider_cache
  • Windows also hits scanner/opencode-related pre-existing failures:
    • core::scanner::tests::collect_session_files_expands_opencode_db_into_virtual_sessions
    • core::scanner::tests::scan_sessions_with_config_hides_disabled_external_cached_sessions
• cargo clippy -p pi-session-cli -- -D warnings is intentionally not claimed for this PR because local green status there depends on separate out-of-scope workaround patches

Example in global search showing all labels (empty search box + "Labels" filter selected):

Example in global search showing label matches (non-empty search box + "All" -- or "Labels" -- filter selected):

Example in session viewer search (non-empty search box matching a node label, "Default" filter selected):

Example in session viewer search (non-empty search box, "Labeled" filter selected):