feat: warn users approaching upload limits and block at 5 GB

[dobby-coder]

Summary

Closes #87.

Adds UI feedback for upload-limit states described in the issue:

• Warning banner on the fileshare page when a user has used ≥3.3 GB of their 5 GB 2-week rolling limit (shown when the limit API reports usage ≥ 2/3 of the cap).
• Hard-block banner when the user has reached the 5 GB limit, with the reset date.
• Per-upload 5 GB error, surfaced inline in the file picker the moment the total selection crosses MAX_UPLOAD_SIZE. Previously the validation dialog only showed it on clicking Send.
• Server-blocked banner rendered when cryptify returns 413 (see Enforce server-side upload limits: 5 GB rolling per email (14 days) cryptify#100). This replaces the generic "Something went wrong" page for rejected-by-quota uploads.

Files

• src/lib/usage.ts — new utility. fetchUsage(email) consumes GET /usage?email=... from cryptify (shape agreed in the coordination note for the upload-limits initiative). Quiet-fails to null if the backend isn't deployed yet. parseLimitExceededBody(body) decodes cryptify's 413 body into a UsageStatus.
• src/lib/components/filesharing/UsageWarning.svelte — debounced banner (500 ms). Renders nothing when usage is below the warn threshold.
• src/lib/components/filesharing/SendButton.svelte — wires in UsageWarning, handles 413 gracefully.
• src/lib/components/filesharing/inputs/FileInput.svelte — adds the inline per-upload error, and the "max 5 GB" text now derives from MAX_UPLOAD_SIZE instead of being hard-coded.
• src/lib/env.ts — adds ROLLING_LIMIT (VITE_ROLLING_LIMIT, defaults to 5 GB), ready for Upgrade max upload size to 5 GB with 5 GB per 2-week rolling limit #85.
• src/lib/locales/{en,nl}.json — new usage.* and fileBox.overLimit* keys; maxSizeText now takes a {max} argument.

Open design question for @rubenhensen

The issue says "proactive check on email input: when the user types their email address, query the backend". The 5 GB limit is tracked per sender email (per cryptify#100). The fileshare UI today has no sender email input — the sender's identity only becomes known after Yivi signing.

This PR wires the proactive check to the first recipient's email as a best-effort hook. It's semantically imperfect (recipient ≠ sender), but it lets the component and the endpoint integration land now. The two proper fixes for follow-up:

1. Add a visible sender email input to the fileshare page, bind UsageWarning to it.
2. Run the usage check after Yivi sign completes — i.e., accept that the warning is post-hoc for a first-time user but proactive for returning users if we cache the last-used sender in localStorage.

Happy to implement either in a follow-up once you pick.

Out of scope

• Thunderbird / Outlook add-ons (mentioned in the issue scope): separate repos. The UsageWarning component and usage.ts utility are deliberately framework-light so an equivalent can be dropped into each addon in follow-up PRs.
• Changing VITE_MAX_UPLOAD_SIZE to 5 GB and setting VITE_ROLLING_LIMIT to 5 GB — that's Upgrade max upload size to 5 GB with 5 GB per 2-week rolling limit #85.

How to verify

npm run check       # passes
npm run build       # passes
npm run dev         # then visit /fileshare

• Drop two files totalling >5 GB → inline error banner appears under the file list.
• Mock cryptify returning usage ≥3.3 GB at /usage?email=... → warning banner renders after typing a recipient email.
• Mock cryptify returning 413 on upload → server-blocked banner renders instead of the error page.

Test plan

• Interactive UI check once backend usage endpoint is reachable (cryptify#100 merged).
• Confirm the sender-vs-recipient question above, then update the UsageWarning binding if needed.
• Follow-up PRs in postguard-outlook-addon and postguard-tb-addon to reuse this component / utility.

Related issues

• Enforce server-side upload limits: 5 GB rolling per email (14 days) cryptify#100 (server-side enforcement and /usage endpoint)
• Upgrade max upload size to 5 GB with 5 GB per 2-week rolling limit #85 (set VITE_MAX_UPLOAD_SIZE=5 GB, VITE_ROLLING_LIMIT=5 GB)
• Add file size limits as primary sales pitch on the frontpage #86 (marketing: communicate limits)

[rubenhensen]

I was mistaken with the proactive check. Check it as soon as you know the senders email.

[rubenhensen]

Superseded by #110 which combines this with #89 and #90.