feat: add preset configurations with built-in and custom presets

Add preset store with three built-in presets (Strict mode, Research only,
Full access) and localStorage persistence for custom presets. Integrate
preset selector into ConfigSidebar with load, save, and delete actions.
Built-in presets cannot be deleted.

Closes #14

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

implementation-plans/issue-014.md

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+---
+---
+
+# Issue #14: Preset configurations
+
+**Status:** COMPLETED
+**Issue:** https://git.shahondin1624.de/llm-multiverse/llm-multiverse-ui/issues/14
+**Branch:** `feature/issue-14-preset-configs`
+
+## Acceptance Criteria
+
+- [x] Save current config as a named preset
+- [x] Load preset from a list
+- [x] Delete custom presets
+- [x] 2-3 built-in default presets shipped:
+  - "Strict mode" — no overrides, restricted tools (FS Write, Run Shell, Run Code, Package Install disabled)
+  - "Research only" — limited tool set for read-only operations (Memory Write, FS Write, Run Code, Run Shell, Package Install disabled)
+  - "Full access" — all overrides relaxed (OverrideLevel.ALL), no tools disabled
+- [x] Presets persisted in localStorage
+- [x] Built-in presets cannot be deleted
+
+## Implementation
+
+### New Files
+- `src/lib/stores/presets.svelte.ts` — preset store with built-in presets and localStorage persistence for custom presets
+
+### Modified Files
+- `src/lib/components/ConfigSidebar.svelte` — added preset selector section at the top with load, save, and delete functionality
+
+### Key Decisions
+- Built-in presets are hardcoded constants, not stored in localStorage
+- Custom presets stored under `llm-multiverse-presets` localStorage key
+- Overwriting a custom preset with the same name is allowed; overwriting built-in presets is not
+- Preset config uses a plain interface (`PresetConfig`) rather than protobuf types for clean serialization
+- Preset section placed at the top of the sidebar before Override Level, matching the issue instructions