Activate dormant surfaces: Gantt/Calendar tabs, trigger preview, webhook panel

[Neonforge98]

Summary

Three features whose backends (and API client wrappers) already existed but had no UI wiring, plus removal of a dead legacy affordance found along the way. This PR activates the dormant surfaces without touching the underlying behavior.

• Gantt and Calendar view tabs — both views were fully implemented (render branches, view-ready data from readWorkItemsViewData, dedicated components) but hard-excluded from WORK_ITEMS_VIEW_TABS. They are now selectable on the project Work Items page; i18n labels already existed in both locales.
• Discussion trigger preview — project_discussion_preview_trigger and its client wrapper existed with zero callers. The comment composer now shows a debounced preview chip: whether the comment will wake the agent, or why not (/note note-only, no linked session).
• Routine webhook management panel — the webhook backend (install/rotate/status/enable, delivery ledger, replay, auto-pause) and all six client wrappers existed with zero UI. The Runs surface now has a Runs | Webhooks toggle; the Webhooks view lists portable routines with install/rotate (one-time secret display with copy), enable/disable, pause state, and the delivery ledger with per-delivery replay. Adds one small Tauri command, project_list_portable_routines, exposing the existing routine_service::list_routines.
• Dead code removal — the legacy orchestrator_start affordance (handleStartAgent, its command constant, the unused collab-lock exposure, and five orphaned i18n keys across all 13 locales) is deleted. The start path of the old orchestrator is superseded by the durable Work Item run pipeline; Retry/Cancel/Accept flows are untouched.

Verification

• npm run typecheck, eslint, and vitest on the touched modules all clean; cargo check and cargo clippy --workspace --all-targets clean (only the pre-existing block v0.1.6 future-incompat warning)
• Live-tested against a dev-build bundle: Gantt/Calendar render real items (Calendar spans multi-day ranges correctly); the trigger preview chip switches between wake and note-only states as you type; the webhook panel was exercised end to end — install, one-time secret copy, real HTTP deliveries (token auth, idempotent dedup, rejected-without-scope and accepted-with-scope both landing in the ledger), replay creating a new run, and both runs visible on the Runs surface