fix(sharing): address xhigh code-review findings on harness sharing

[DIodide]

Follow-up fixes from an extra-high-effort (/code-review xhigh) recall-mode review of the harness-sharing feature merged in #146. Nine finder angles → per-finding verification → sweep surfaced these; duplicates and non-triggerable/intentional-parity items were dropped.

Correctness

• editSharedHarness blank-out — a less-trusted editor-recipient could BLANK the owner's name/model by clearing the field and saving (only an upper bound was enforced). Empty/whitespace name/model are now ignored server-side, and the Edit dialog disables Save when either is empty.
• harnesses.remove orphaned grants — deleting a shared harness left its harnessShareGrants rows behind. They were un-revokable (revoke re-asserts ownership via the now-deleted harness) and a stale public token kept resolving to a dangling id. remove now cascade-deletes the grants.
• hasAuth mislabeled — publicHarnessProjection.hasAuth was Boolean(authToken), but oauth/tiger_junction servers DO require auth while keeping their secret off the harness row, so the public viewer showed no "requires auth" badge for them. Now derived from authType !== "none" (leaks nothing new — authType is already in the projection).
• double clone — on the share viewer, the manual Clone button and the auto-resume effect could both fire before isPending propagated, creating two "Copy of X" harnesses. requestClone is now guarded by an in-flight ref.
• sessionStorage crash — the /harnesses claim effect read/wrote sessionStorage outside any try/catch; a sandboxed/partitioned context (Safari private mode, embedded iframe) threw synchronously and broke the page. Now guarded.

Polish

• EmptyState flash — /harnesses no longer flashes the empty state while the incoming-shares query is still in flight (it often resolves after harnesses.list).
• clone-intent leak — the owner-redirect path on the share viewer now clears the pending clone intent so it doesn't linger in sessionStorage for its TTL.
• listMySharedHarnesses order — now sorted newest-first, matching the adjacent listMySharedConversations on the Manage Sharing page.
• dead arg — dropped the unused grantId arg on editSharedHarness (authz is resolved via the bound grant; the arg falsely implied a grant-scoped edit).

Deliberately skipped (noted, not fixed)

• Re-invite of an already-bound recipient creates a duplicate grant — the proper fix reworks the adversarially-reviewed bind/claim flow and its display; practical harm is low (duplicates self-merge on the invitee's next claim, and the owner can demote directly via the existing role toggle on the bound row).
• ensureHarnessPublicLink refreshes owner name/avatar to undefined when called without a profile — intentional parity with chat-share's documented "refresh snapshot on re-open"; not a divergence worth introducing.
• lastAccessedAt only bumped on clone, not view — no UI surfaces the field today; pure naming/semantic, no wrong output.
• conversations.remove has the same orphaned-shareGrants omission — real, but out of scope for this harness-sharing PR (different feature).

Validation

• Convex: codegen ✓, check-types ✓, 218 tests ✓ (+2 regression tests: empty-field guard, remove cascade)
• Web: biome ✓, tsc 21/21 baseline (no new errors in touched files) ✓, 239 tests ✓
• No FastAPI changes.