fix(dashboard): preserve user policy edits - #7469

Summary

• track the last server form separately so background refetches refresh clean forms without overwriting unsaved edits
• rebase only locally edited leaves onto newer server forms so unrelated concurrent server changes survive the next full PUT
• make a successful save the clean baseline without hiding edits made while the request was in flight
• disable both Save controls when the form is unchanged
• normalize server-provided channel keys before duplicate checks and payload editing
• remove unreachable empty-entry validation and its dead locale key from all five dashboard locales
• add regressions for dirty and clean refetches, concurrent server fields, saved baselines, mixed-case channels, and both Save controls

Verification

• corepack pnpm exec vitest run src/pages/UserPolicyPage.test.tsx (14 passed)
• corepack pnpm test (103 files, 1101 tests passed)
• corepack pnpm typecheck
• corepack pnpm lint
• corepack pnpm build
• corepack pnpm test:i18n-parity (expected main-branch baseline failure only: the existing 8 extra plural keys in both pl.json and uk.json; ko.json and zh.json pass)
• git diff --check origin/main...HEAD

Out of scope

• daemon policy validation and channel adapter naming remain unchanged
• blank lines continue to follow the documented server-compatible trim-and-drop behavior
• baseline Polish and Ukrainian plural-key drift remains for a separate change