fix(admin): serialize unset config fields using their defaults

[jmagar]

• serializeConfig now falls back to a field's default (with type-appropriate last resorts: 0/false/"") instead of emitting null for unset number/boolean/text values
• Admin setup failure now surfaces the real error message instead of a generic "Try again."
• Adds a test covering effective-default serialization
• Includes rebuilt bundled frontend assets (public/)

[jmagar]

Applied fixes from the three converging review passes (commits 58b35089, d928df80):

Critical semantics fix — serializeConfig in web/src/screens/admin.tsx no longer invents 0/false/"" for fields with no value and no default. Those keys are now omitted from the serialized payload entirely, matching the Go server's ApplyPatch (server/common/config.go:325), which skips absent keys and preserves existing/dynamic-default values via Configuration element Interface(). This still fixes the original fresh-install bug (no more null values aborting ApplyPatch) without the side effect of silently pinning real settings like force_ssl=false on save. Falsy-but-set values (false, 0) and enable-type defaults are preserved correctly.

Error message guards — replaced the remaining unguarded (error as Error).message casts (session load, setup, save) with the instanceof Error idiom already used elsewhere in the file, each with a sensible fallback string for non-Error rejections.

Test coverage — extended admin.test.ts with cases for: value+default both unset (omitted), falsy-but-set boolean over a truthy default, zero-but-set number over a non-zero default, enable-type default resolution, and omission for both boolean and text types. Updated the existing "effective defaults" test since host (null value, null default) is now omitted rather than coerced to "". Added two component-level tests in admin-screen.test.tsx covering the setup error path for both Error and non-Error rejections.

All touched tests pass (14/14), plus tsc --noEmit and eslint on the changed files are clean.

Also rebuilt and staged public/ via make frontend-stage so the embedded assets match the updated admin.tsx bundle (small, expected diff — two chunk renames plus updated HTML references).