feat: rewrite Improv tool — scenario-first tavern flows, cross-entity context, LLM-backed generation with fallback

[jmwashi]

Motivation

• Replace a generic, mode-based improv prompt with a scenario-first flow (tavern, bartender, suspicious patron, rumor, complication) so DMs get immediate, table-ready seeds.
• Allow the improv tool to pull facts from across the world model (not just items) and default to session/scene references for quick reuse.
• Centralize context assembly and generation logic so multiple UIs can reuse the same synthesis and LLM/fallback behavior.
• Provide structured LLM output (schema-first) while guaranteeing usable fallback beats when no provider/config is available.

Description

• Replaced the session improv UI with a scenario-first tavern flow in src/renderer/src/pages/Sessions/ImprovToolTab.tsx including input fields and DM-ready output cards for dialogueOpener, motive, secret, escalationBeat, and fallbackBeat.
• Added a cross-entity context selector (Characters, Locations, Factions, Items, Lore, Notes, Scenes) that is campaign-filtered and defaults to session referenced entities + active scene via getDefaultImprovEntityRefs in src/renderer/src/lib/improv/context.ts.
• Introduced shared improv utilities under src/renderer/src/lib/improv/: types.ts (contracts), context.ts (context synthesis and formatting), and generate.ts (LLM integration with JSON-schema expectations and fallbackGenerate for robust fallback outputs).
• Integrated generation wiring into the session UX by adding an Improv tab to the session sidebar and rendering ImprovToolTab from SessionDetail, and added lightweight defensive typing/formatting tweaks to the new files.

Testing

• Ran npm run lint (executed repository lint) and it failed due to pre-existing, repository-wide lint/type style issues outside the scope of this PR; failures are unrelated to the improv feature.
• Ran npm run typecheck and both node and web type checks passed successfully.
• Verified the renderer dev server with npx vite and captured a Playwright screenshot of the improv tab UI; Electron runtime failed to launch in this environment due to a missing system library (libatk-1.0.so.0), so full Electron smoke-test was not possible.
• Validation notes: the LLM-available path was inspected to confirm the request uses configured VITE_LLM_* or local improvTool.llmConfig, requests a json_schema response, and only accepts outputs that satisfy required keys (returns usedFallback: false and diagnostics on success); the fallback path was validated by code paths for missing config, non-OK HTTP responses, missing/malformed content, schema mismatch, and request exceptions, and always produces complete, DM-usable beats via fallbackGenerate.

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