feat: add prestige system and skill upgrades with comprehensive documentation

docs/task7/plan_skillstab.md

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+# Refactor Plan: SkillsTab.tsx
+
+## Current State
+- **File:** `src/components/game/tabs/SkillsTab.tsx`
+- **Lines:** 434
+- **Target:** ≤400 lines (reduce by ~34 lines)
+
+## Proposed File Structure
+
+### 1. `src/lib/game/utils.ts` (NEW, ≈20 lines)
+**Contains:**
+- `formatStudyTime` function
+
+**Rationale:** Simple pure utility function, zero-risk extraction.
+
+### 2. `src/components/game/tabs/SkillUpgradeDialog.tsx` (NEW, ≈80 lines)
+**Contains:**
+- `SkillUpgradeDialog` component (extracted from `renderUpgradeDialog`)
+- Dialog UI and selection state handlers
+
+**Rationale:** Isolates the upgrade dialog UI (~100 lines worth from parent), simplifies SkillsTab significantly. Uses callback props.
+
+### 3. `src/components/game/SkillRow.tsx` (NEW, ≈100 lines)
+**Contains:**
+- `SkillRow` component for individual skill rows
+- Level dots, buttons, study toggle, tier-up, milestone badges
+
+**Rationale:** Encapsulates per-skill UI (~150 lines worth from parent), reusable, simplifies SkillsTab.
+
+### 4. `src/lib/game/hooks/useSkillUpgradeSelection.ts` (NEW, ≈40 lines)
+**Contains:**
+- `useSkillUpgradeSelection` custom hook
+- Selection state and mutation logic for milestone upgrades
+
+**Rationale:** Encapsulates upgrade selection logic previously inline in SkillsTab.
+
+### 5. `src/components/game/tabs/SkillsTab.tsx` (≈150 lines)
+**Keeps:**
+- Core `SkillsTab` component shell
+- Category-level layout
+- Main store hook calls
+- Tab switching
+
+**Rationale:** Maintains coordination role while delegating details to extracted components/hooks.
+
+## Circular Import Risks
+
+**MEDIUM RISK:**
+- `SkillRow` needs access to many store selectors and actions. May accept callbacks as props to avoid direct store access (kept in parent).
+- `SkillUpgradeDialog` needs data from store; will receive computed values as props.
+- `useSkillUpgradeSelection` needs `SKILL_EVOLUTION_PATHS` and store commits - safe.
+
+**MITIGATION:**
+- Keep store interactions in `SkillsTab`, pass data down via props.
+- Accept slight prop drilling vs. direct store access in extracted components for cleaner boundaries.
+
+## Extraction Order
+
+**1 → 4 → 3 → 2 → 5**
+
+1. Create `utils.ts`, move `formatStudyTime`, update SkillsTab import.
+2. Create `useSkillUpgradeSelection` hook, move selection logic, update SkillsTab.
+3. Create `SkillRow`, move per-skill UI (pass callbacks from parent), update SkillsTab.
+4. Create `SkillUpgradeDialog`, move dialog UI, update SkillsTab.
+5. Delete old extracted sections from SkillsTab, verify ≤400 lines.