feat: clearer feedback across the identification/save flow

[gwado]

⚠️ New feature, not a bug fix. This changes existing app behavior — feel free to close if it's out of scope for the project.

Summary

Three related UX improvements to the identify → results → save flow:

• RequestPage now stays visible with its own loading indicator while an identification request is in flight, instead of popping back to MainPage immediately and showing a generic spinner there with no visible connection to what triggered it. On success it pops itself and pushes ResultsPage directly; on error it stays put so the user can retry without re-adding all their photos.
• Identification::identifyPlant() now classifies failures instead of always reporting a raw "(err/code)" pair: invalid/rejected API key (401/403), quota exceeded (429), server error (5xx), unreachable network, or a generic fallback, each with a clear message. The identificationResult signal gained an errorCode alongside the existing message so QML can react differently per case — in particular, an invalid-key error now offers to open Settings directly from the error dialog, instead of only surfacing this after the user has already taken photos and started a request.
• Saving a result now shows a brief "Plant saved" toast before returning to the saved-plants list, instead of popping back to MainPage with no visible confirmation that the save succeeded.

Type of change

New feature

Test plan

• clickable build --arch arm64 builds cleanly; the produced package passes clickable's click-review.
• Built and installed on a physical Pixel 3a (arm64): ran a full identification request end-to-end, confirmed the loading indicator now stays on the request page, and confirmed the save confirmation toast appears before returning to the saved-plants list.
• The specific error-classification branches (invalid key, quota, server error, offline) were validated by code review/tracing, not reproduced live against Pl@ntNet's API on-device (would require deliberately breaking each condition).

Notes

• No user-facing strings changed in po/ in a way that needs an update for this PR — the new "Plant saved" string and the four new error messages are not yet in po/plants.s710.pot; happy to run the translation template update if you'd like it included here.