docs: add lnbits frontend gotchas + extension-dev reference

Two new docs to round out the lnbits-specific reference set:

docs/lnbits-frontend-gotchas.md — the Vue/Quasar UMD traps that don't
manifest under the build-step model most tutorials assume. Sections:

- No self-closing tags. UMD-loaded templates are parsed by the
  browser's HTML parser, which doesn't honor self-close on non-void
  elements; nesting silently breaks. Fine in .vue SFCs because the
  build step rewrites them — but lnbits templates aren't SFCs.
- CSS specificity vs Quasar utilities. LNbits applies !important
  overrides on .text-caption / .text-grey-*; class-based rules in
  extension pages lose. Reach for inline :style bindings or static
  style="" attrs instead.
- Cache busting via ?v={server_startup_time}. Bumping JS requires a
  server restart; templates re-render every request.
- Dark-mode color discipline. bg-{color}-1 utilities don't get a
  default text-inversion in dark theme; pair every pale background
  with an explicit text class.

docs/lnbits-extension-dev.md — auth + testing + migration pattern.
Sections:

- Auth decorators table: require_invoice_key, require_admin_key,
  check_admin, check_super_user. The require_admin_key vs
  check_admin distinction is the most common misuse — one is
  wallet-level write access (any user), the other is instance admin.
- Testing: FakeWallet for CRUD/API/UI, regtest only for real
  Lightning behavior. Why the dev CLI defaults to --fakewallet.
- The migrations_fork.py pattern: keep migrations.py byte-identical
  to upstream, put fork-only schema deltas in a sibling file loaded
  under <ext>_fork in dbversions. Covers: architecture facts
  (dbversions in core DB, no cross-DB atomicity → idempotent
  migrations mandatory), squash recipe for adoption, one-time
  dbversions surgery for installs that previously ran old fork
  migrations, the upstream-overlap rebase scenario.

Both linked from README "Further reading". No personal identity in
either file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -239,8 +239,18 @@ lb fix-issue-123   # cd ~/dev/lnbits/fix-issue-123
 - [`docs/lnbits-workspace-notes.md`](docs/lnbits-workspace-notes.md) —
   practical gotchas and design constraints for day-to-day lnbits work:
   port choice, `LNBITS_SRC` build-context traps, the
-  extension-folder-upgrade-wipes-fork issue, Nostr key handling,
-  CLINK protocol scope, fork versioning.
+  extension-folder-upgrade-wipes-fork issue, settings precedence
+  (`.env` vs DB), Nostr key handling, CLINK protocol scope, fork
+  versioning.
+- [`docs/lnbits-extension-dev.md`](docs/lnbits-extension-dev.md) —
+  building and maintaining LNbits extensions: auth-decorator
+  distinctions, FakeWallet vs regtest testing strategy, the
+  `migrations_fork.py` pattern for keeping fork-only schema deltas
+  out of upstream-tracked `migrations.py`.
+- [`docs/lnbits-frontend-gotchas.md`](docs/lnbits-frontend-gotchas.md) —
+  Vue/Quasar UMD traps in lnbits page templates: no self-closing
+  tags, CSS specificity vs Quasar's `!important` utilities, cache
+  busting via `?v={server_startup_time}`, dark-mode color discipline.
 
 ## Contributing to this scaffold