chore: drop *.xcscheme gitignore rule and track Savely scheme

[Ivan-LB]

Summary

• Delete the *.xcscheme line from .gitignore (line 18).
• Track Savely.xcodeproj/xcshareddata/xcschemes/Savely.xcscheme (was silently ignored before).

Why

Discovered while implementing Task 2 of docs/plans/maintenance-cleanup.md: the *.xcscheme rule had been globally ignoring every scheme file since the repo started. The existing Savely.xcscheme on disk was never tracked — git ls-files | grep xcscheme returned empty. CI's xcodebuild -scheme Savely worked only because xcodebuild auto-derives schemes when the file is missing.

Two consequences:

1. CI's build was non-deterministic (auto-derived scheme could drift from the developer's local one).
2. Any future shared scheme created via Xcode's "Manage Schemes → Shared" checkbox would have been silently dropped from commits — a real footgun.

Dropping the rule fixes both: the existing scheme becomes tracked verbatim, future shared schemes (e.g. SavelyTests.xcscheme if we ever want to run unit tests independently of UI tests) can commit naturally. User-specific scheme state continues to be excluded via xcuserdata/ (lines 12 and 22).

This implements Option B from the plan, with Path 2 expansion (track scheme alongside gitignore change) since leaving the scheme untracked would reproduce the same opaque-state problem.

What's in the scheme

Standard Xcode-generated scheme (LastUpgradeVersion 1540, version 1.7):

• Build action: Savely.app only
• Test action: both SavelyTests and SavelyUITests, parallelizable = YES
• Run/Profile/Analyze/Archive: defaults pointed at Savely
• No hardcoded simulator UDIDs, no env vars, no command-line args, no secrets

Test plan

• xcodebuild build -scheme Savely -destination 'platform=iOS Simulator,name=iPhone 17 Pro' — passes
• xcodebuild test -scheme Savely -destination 'platform=iOS Simulator,name=iPhone 17 Pro' -skip-testing:SavelyUITests — passes
• git ls-files | grep -i xcscheme now lists Savely.xcodeproj/xcshareddata/xcschemes/Savely.xcscheme
• git status clean (only docs/ untracked, separate concern)
• Savely/Config.plist and Savely/GoogleService-Info.plist correctly stay gitignored

Risks

Minimal. Note for reviewer:

• The Test action lists SavelyUITests with skipped = NO, but .github/workflows/ci.yml overrides this with -skip-testing:SavelyUITests (intentional per commit 33b5f5a — template launch-perf tests flake on shared CI runners). That mismatch is by design and out of scope for this PR.
• Local builds validated on iPhone 17 Pro instead of iPhone 16 Pro (CLAUDE.md / CI default) because Xcode 26 no longer ships the 16 Pro simulator locally. CI's macos-15 runner still has it, so CI runs unchanged.

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