feat: restrict view-mode click activation to block content (not margins)

[oobagi]

Problem

In view mode, clicking anywhere on a block's rendered Y-row activates it — including horizontal margin areas outside the block's actual content. This leads to accidental checklist toggles and embed-block opens when a user is just clicking around to focus the terminal or select text.

Context

• Click handling lives in internal/editor/editor.go:1510 (tea.MouseClickMsg case), with view-mode branch at line 1538.
• Block lookup uses m.blockIndexAtLine(hoverY) (line 1541) which maps a Y coordinate to a block but does not check X. Any X within the terminal width on that row resolves to the block.
• Clickable blocks in view mode today: Checklist (toggle at line 1546), Embed (open at line 1558). Definition hyperlinks (Definition hyperlinks in text #211) likely add more.

Approach

Two options — the issue is asking to pick one:

Option A — Margin guard (recommended default). Compute each block's actual rendered content X-range (respecting centered view-mode layout, padding, indent). Only activate the click if msg.X falls inside that range. This is the least-surprising behavior and doesn't require user configuration.

Option B — Config toggle disable_clickable_items. Add a bool config + runtime toggle that turns off click activation entirely in view mode. Useful for users who just want read-only no-interaction view mode.

Recommend doing A first (fixes the main complaint), and optionally adding B later if users still want full opt-out.

Tasks

• Expose (or compute on demand) the content X-range for each rendered block in view mode. View mode centers content, so the range depends on terminal width and content width.
• In the MouseClickMsg view-mode branch, bail out early if msg.X is outside the content range.
• Preserve existing hover behavior (hover visual feedback can stay row-wide, or also be narrowed — decide).
• (Optional) Add disable_clickable_items config with a runtime toggle.
• Tests: clicks in margin don't toggle; clicks on content still toggle; centered layout boundaries correct at narrow/wide widths.

Acceptance criteria

• Clicking in the left/right margin of a checklist block does NOT toggle it.
• Clicking on the actual checklist text/box still toggles it.
• Same guard applies to embed blocks and any other view-mode-clickable types.
• No regression in hover highlight behavior (or deliberate, documented change).