feat: add privacy-safe remote acceleration settings and status

[Abhash-Chakraborty]

Summary

Add frontend controls, consent, connection testing, and visible status for optional self-hosted remote ML acceleration.

Why this is required

Remote processing can help low-power clients, but it transmits sensitive image data. Users must deliberately enable it and always know when it is active.

Where to work

• docs/plans/not-started/remote-acceleration.md — required privacy and UX behavior
• frontend/src/app/settings/page.tsx — settings integration
• frontend/src/components/ — consent, warnings, and active-mode badge
• frontend/src/lib/api.ts — typed configuration/test calls
• existing settings tests under frontend/src/__tests__/

This UI depends on backend contracts from #260. Do not invent endpoint shapes independently.

Acceptance criteria

• Remote mode is off by default with no endpoint preconfigured.
• First enable requires explicit informed consent.
• Users can configure URL, credential, allowed features, and EXIF stripping.
• Insecure HTTP and known tunnel risks produce clear warnings.
• Connection testing reports actionable success/failure states.
• Processing/search/gallery surfaces visibly indicate remote mode.
• Settings persist through the approved configuration API.
• Tests cover consent, warnings, persistence, and default-off behavior.

Validation

Run frontend checks/tests/build and verify both light and dark themes.

Out of scope

Do not add a project-hosted endpoint or silently transmit any user data.

[Abhash-Chakraborty]

Split out a beginner-friendly UX-copy documentation starter task from this larger remote acceleration UI issue:

• docs: add remote acceleration UX copy checklist #268

[YashKrTripathi]

Hi! @Abhash-Chakraborty I'm a GSSoC'26 contributor and I'd like to work on this issue.

My approach:

1️⃣ Settings UI — Remote Acceleration Section

• Add a new "Remote Acceleration" section in the Settings page under frontend/src/app/
• Fields/controls:
  • Enable/Disable toggle (off by default — no remote endpoint preconfigured)
  • Server URL input (with format validation)
  • API Key input (masked, with show/hide toggle)
  • Feature checkboxes — e.g. embeddings, clustering, search (granular per-feature opt-in)
  • EXIF stripping toggle (default ON for privacy)
  • "Test Connection" button → calls a helper in frontend/src/lib/api.ts and shows success/failure inline

2️⃣ First-Enable Consent Dialog

• When the user flips the enable toggle for the first time, open a modal that:
  • Explicitly states images may be sent to the configured user-owned server
  • Clearly distinguishes full image transmission vs vector-only clustering (different data exposure levels)
  • Requires the user to check an acknowledgment box before "Enable" is clickable
  • Cannot be bypassed by reloading — consent persisted in app config

3️⃣ Endpoint Safety Warnings

• Non-localhost HTTP warning: if URL is http:// and host ≠ localhost/127.0.0.1, show a blocking warning requiring explicit acknowledgment (insecure transport for image data)
• Cloudflare Tunnel hostname warning: detect known CF Tunnel domains (*.trycloudflare.com, etc. per docs) and surface a clear warning per the documentation guidance
• Both warnings must be acknowledged before save

4️⃣ Active Remote-Mode Indicators

• Add a small, persistent badge/icon (e.g. "☁ Remote Mode") visible during:
  • Processing flows
  • Search results
  • Gallery browsing
• Place it consistently (e.g. header / status bar) so users always know when their data path is remote
• Reuse a shared <RemoteModeBadge /> component for consistency

5️⃣ Persistence & API Layer

• Persist all settings via the app's existing config mechanism (no new storage layer)
• Centralize the remote-mode helpers (isRemoteEnabled, getRemoteConfig, testConnection) inside frontend/src/lib/api.ts
• Ensure feature flags drive whether each pipeline (embedding/clustering/search) actually uses the remote endpoint

6️⃣ Documentation

• Update docs/plans/not-started/remote-acceleration.md → move/update relevant sections to reflect the implemented UX (consent flow, warnings, indicators)

7️⃣ Testing

• Manual verification of every acceptance criterion (consent gate, warnings, indicators, persistence, default-off)
• Verify no remote endpoint is preconfigured on a fresh install
• Confirm settings survive reloads and respect the app's config mechanism

Scope guarantees:

• Frontend-only changes (settings UI + indicators + warnings) — no backend modifications
• Privacy-first: explicit consent + zero preconfigured remote endpoints

ETA: 3 days. Please assign this to me!