Fix 403 game lookups by not reusing cached client User-Agent (#89)

[misantronic]

Fixes #89.

Problem

The proxy identified itself to RetroAchievements with ua::last — the User-Agent of whichever emulator most recently passed through it. RA reads the first token of the User-Agent as the client identity and answers blocked clients with 403 unsupported_client on every endpoint, including r=gameid.

So one emulator RA rejects poisoned every request the proxy made on its own behalf, for every console. The reporter's Game Boy folder scan failed because a GameCube emulator's identity was cached. ua::last is exempt from cache eviction, so it never expired.

Worth noting: RA sets clientVersion = 'Unknown' when it can't parse a version out of the User-Agent, and that sorts below every minimum, so a current build with a versionless UA is rejected exactly like an ancient one.

Fix

New self_user_agent() used for every request the proxy originates — manual ROM caching, preview images, the reachability probe, and the background refresher. Genuinely forwarded emulator requests still carry the client's real User-Agent via build_forward_headers, and pending awards still replay under the User-Agent that earned them.

Storage.load_user_agent is removed; it existed only to feed this bug.

Diagnostics

http_get was discarding RA's response body, so the only thing that ever surfaced was HTTP Error 403: Forbidden. It now extracts RA's own Code/Error and logs the outgoing User-Agent:

before: GET failed status=403 reason=Forbidden url=...
after:  GET failed status=403 reason=unsupported_client: This client is not supported. ua=... url=...

Verification

Confirmed against the live RA API using the exact hash from the report:

RetroArch/1.21.0 (Linux) RAOfflineProxy/Linux/...  ->  {"Success":true,"GameID":504}
Dolphin/2407 RAOfflineProxy/Linux/...              ->  403 unsupported_client

Reporter confirmed the test build resolves the issue on ROCKNIX.

Existing installs self-heal on upgrade — the poisoned row is simply no longer read.

Notes

• PROXY_UA_TAG now derives from APP_VERSION instead of being a second hardcoded copy that had drifted.
• APP_VERSION is bumped to 1.10.0-alpha7. It is shared by the muOS, Knulli and Onion bundles, whose own build scripts still default to alpha2 filenames.
• Android has the same defect (RomScanner.kt, MainViewModel.kt, AwardFlusher.kt, ProxyService.kt) and is not addressed here.