[UI-5] Create DashboardPanel class (WebviewPanel)

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Context

Part of UI-5 – Dashboard — Overview + Sessions — see vscode-ui-implementation-plan.md § UI-5.

What

Create src/webview/dashboard/panel.js with a singleton pattern:

• DashboardPanel.createOrShow(context) — creates a WebviewPanel in the editor area or focuses the existing one.
• retainContextWhenHidden: true — preserves JS state across tab switches.
• Column: vscode.ViewColumn.One (or beside, per plan).
• HTML shell mirrors the prototype topbar + tab layout (index.html L2100–L2160).

Acceptance criteria

• Only one DashboardPanel exists at a time.
• Closing and reopening restores the last active view.
• Panel title updates to OctoClock Dashboard.

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Note

The previous stub at webview/dashboard/panel.js was rewritten with the singleton pattern. Method named open(context) (matches the existing convention in this codebase and the implementation plan example, both of which use open not createOrShow).

Singleton & focus

sequenceDiagram
    participant U as User
    participant Cmd as octoclock.openDashboard
    participant DP as DashboardPanel
    U->>Cmd: trigger
    Cmd->>DP: open(context)
    alt currentPanel === undefined
        DP->>DP: new DashboardPanel(context)
        DP->>DP: createWebviewPanel('OctoClock Dashboard', ViewColumn.One, { retainContextWhenHidden: true })
    else
        DP->>DP: currentPanel._panel.reveal()
    end

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State preservation

retainContextWhenHidden: true means the active tab, range, week offset, and filters are preserved when the user switches editor tabs and back. The webview only re-runs its bootstrap when the panel is fully closed and reopened, at which point the host pushes a fresh init message.

Verification

• pnpm run build:vscode ✓ (dashboard/app.js — 12.72 kB)
• pnpm test → 185/185 ✓
• pnpm exec biome check → no errors on dashboard files