fix: keep in-progress draft and class-fit settings while editing

[usehoplite]

Summary

Two papercuts found while auditing the main teacher flows in the workspace UI, plus the button that depended on the old behavior:

1. Editing profile fields discarded the in-progress draft. updateProfile() reset worksheet, hasDraft, lastAgentMessage, and lastError on every keystroke in Grade/Subject/Topic. Typing one character into the Subject field silently wiped all generated questions (and reset the reading-target constraint back to the default grade value). It now only syncs the reading-level constraint when the grade is explicitly changed, and leaves the draft and constraint state untouched otherwise.

2. "Remove source" wiped the whole workspace. removeSource() delegated to initialState(), which cleared not only the source material but also the class-fit constraints (time limit, reading target, question mix, standards) and the entire activity history. It now clears only source, the worksheet draft, and draft/agent/error flags, and appends a "Removed source material · class fit kept" activity entry so the shared history stays intact.

3. "Clear workspace" still fully resets. The activity panel's clear button previously called removeSource(); since that action is now scoped, the button routes to the store's pre-existing reset() action, so it still empties profile, constraints, activity, and drafts as before.

Also includes .hoplite/settings.json: records the verified dev-server setup/run commands and preview port for sandboxes that provision this repo, and installs git-lfs (required by the first-party PR publishing path).

No changes to WebMCP tool registration, source handling, browser support, or deployment configuration.

Verification

• npm run lint — 0 warnings, 0 errors
• npm test — 7/7 pass
• npm run build — succeeds (dist produced)
• Live browser confirmation (dev server, seeded Maths/Fractions source):
  • Generated a draft, typed a character into Subject → all 6 question cards, live constraint checks, and the raised reading target remained present (previously the draft vanished).
  • Removed the source → subject/topic, reading target (Grade 4.5), time limit, and 3 prior activity entries survived; only the source preview and cards cleared.
  • Clicked "Clear workspace" → profile, reading target, activity, and cards all reset to defaults.

Screenshots are not included: the active model route cannot inspect image pixels, so I can't vouch that a capture shows the intended state; the behavioral evidence above is from live DOM/text inspection of the running app.