SQLite and LibreDB providers report databaseSizeBytes as 0 when the size is unknown

`DatabaseOverview.databaseSizeBytes` is optional on purpose: absence and zero are different facts (see the docblock in `src/lib/db/types.ts`). The two embedded providers get this wrong by coercing an unreadable size into a literal `0`: the size initializer in `src/lib/db/providers/sql/sqlite.ts`, and `fileSizeBytes()` in `src/lib/db/providers/embedded/libredb.ts`, which returns 0 when `statSync` throws. `StorageTab.tsx` keys its whole breakdown UI off `databaseSizeBytes !== undefined`, so a fabricated 0 does not hide a number, it renders a breakdown over a zero-byte database.

**Scope.** This issue covers only the two embedded engines, SQLite and LibreDB, which need no database server to test. The other providers in the backlog entry are out of scope here.

**Fix.** Make `databaseSizeBytes` absent when the size genuinely cannot be read, keep a real 0 reading as 0, update `docs/providers/sqlite.md` and `docs/providers/libredb.md`, and add a test per provider pinning both arms (the sibling `activeConnections` fix is the pattern to mirror).

**Done when** both providers, their docs and their tests move together in the same PR, per the provider triad rule.

Context: `docs/BACKLOG.md`, entry D44.

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