Add a client-side persistence-status layer with save-failure recovery

[kmcginnes]

Description

Graph Explorer stores all user data client-side in IndexedDB. Previously, when a durable write failed, in-memory state silently diverged from disk and was lost on reload — with no signal to the user.

This adds a persistence-status layer that owns write failure centrally. Every IndexedDB write now routes through a shared retry/coalescing queue that classifies failures, retries transient ones with backoff, and reports terminal failures into a single global status store. A "Changes not saved" indicator surfaces in the nav bar only when a write has terminally failed; clicking it opens a dialog with the failure details and — when storage is full (and therefore still readable) — a button to back up the configuration to a file.

The persisted setter now returns void: callers just set state, and the storage layer is solely responsible for getting it to disk. This composes with the cross-tab merge work (ADR per-key-diff-merge-cross-tab-reconciliation) at the same flush seam without conflicting.

How to read

1. classifyStorageError.ts — start here; the pure error taxonomy (quota/access terminal, everything else retryable)
2. persistenceStatusStore.ts — the global external store (idle | saving | failed) outside React/Jotai
3. writeQueue.ts — per-key serialize + coalesce-to-latest + retry/backoff, knows only a flush thunk
4. index.ts — wires the singleton store + queue into persistThroughQueue
5. writeThroughAtom.ts — the setter-returns-void change (shared by the localForage and active-connection paths)
6. PersistenceStatusIndicator.tsx — the nav-bar indicator + detail dialog
7. ADR — the design rationale and rejected alternatives

Core vs supporting: the four persistence/ modules + the indicator are the feature. The route files (Connections, DataExplorer, GraphExplorer, SchemaExplorer, SettingsRoot), NavBar spacing, and ConnectionDetail debug actions are wiring. The test-helper and *.test.ts changes are the mechanical fallout of the setter returning void (they synchronize on waitForIdle() instead of awaiting a per-write promise).

Validation

• pnpm checks passes (lint, format, types).
• pnpm test passes (1854 passed, 1 expected-fail — the pre-existing Concurrent edits in multiple browser tabs silently clobber shared persisted collections (styling, schema, connections, sessions) #1820 cross-tab clobber regression marker, unchanged).
• New unit tests cover the error taxonomy, status store (aggregation precedence, per-key clear, waitForIdle, reset), and write queue (coalescing, retry-then-succeed, terminal no-retry, escalation after cap, per-key independence), plus a component test for the indicator.
• Manual: enable debug actions (Settings → General), open a connection's detail screen, and use "Force Save Failure (Out of Storage / Storage Blocked)" and "Reset Save Status" to exercise the indicator and dialog through the real pipeline.

Related Issues

• Closes Storage layer owns persistence failure: global save-status indicator + retry #1857
• Related to Back up from in-memory state so recovery includes the failed write #1858 (follow-up: back up from in-memory state so recovery includes the failed write)

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.