feat: add Polish (pl) locale and localize hardcoded email viewer labels

[priard]

Adds Polish language support.

Changes

• New locales/pl/common.json with full key parity vs EN
• Adds polish + switch_to_polish keys to all 10 existing locales (de, en, es, fr, it, ja, nl, pt, ru, uk)
• Wires pl into i18n routing and language switcher
• Localizes hardcoded "Reply" / "Reply all" / "Forward" labels in email viewer

Verification

• npm run typecheck ✅
• JSON validation ✅
• EN/PL key parity ✅
• Switched EN→PL→DE→PL, verified email viewer updates without reload ✅
• Manual smoke test of the language switcher and email viewer ✅

Screnshots

[boris22100]

Hi @priard ! why don't have a look about https://github.com/bulwarkmail/webmail

There's Polish already included :

[priard]

Hi @boris22100, thanks for the pointer, this looks really interesting, I’ll definitely have a look.

[antedebaas]

i found Bulwark to be unstable. lots of Ai prompting and little testing ahead of release.

[boris22100]

@antedebaas Tell us if JMAP Webmail works better for you ?

[antedebaas]

Its simply a more stable product.
bulwark constantly adda features with clearly ai.
And due to that the project is starting to become an mess of features.
It also breaks other parts of the project which i then have to file a bug report for.
This then gets resolved with an entire new feature written by ai.

Also bulwark sends a lot of telemetry jmap-webmail does not.