feat(commands): add /explain slash command with /screen and image support

[quiet-node]

Summary

• Adds /explain slash command: explains any concept, term, or code snippet in plain language with a concrete example
• Extends /screen + utility command combos: utility commands (e.g. /screen /explain) now build a composed prompt override so the screenshot is used as $INPUT instead of sending raw trigger strings to the model
• Adds image fallback for utility commands: /explain (and other non-translate utility commands) with only an attached image build a synthetic prompt using "the attached image" as $INPUT when no text is present

Changes

• src/config/commands.ts — new /explain entry in the COMMANDS registry with prompt template, docs, and prompt help metadata
• src/components/CommandSuggestion.tsx — EXPLAIN_ICON SVG and iconForTrigger case for /explain
• src/App.tsx — handleScreenSubmit accepts promptOverride?; submit handler builds screenPromptOverride for /screen + utility combos; image fallback for utility commands; CONTROL_CHARS extracted to module-level constant (DRY fix)
• src/__tests__/App.test.tsx — tests for /screen /explain, no-text image fallback, and /screen /translate passthrough behavior
• docs/commands.md and src-tauri/prompts/generated/slash_commands.txt — documentation and prompt metadata updated

Fixes #134

Test plan

• /explain what is a closure? — returns plain-language explanation with example
• /explain with highlighted code selected — uses selected text as $INPUT
• /screen /explain with no other text — captures screen, explains screenshot content
• /screen /explain this error — captures screen, uses "this error" as additional instruction
• /explain with attached image and no text — uses "the attached image" as synthetic input
• /explain with no text and no image — no submission (correct early exit)
• /screen /translate French — screen captures but sends raw query (translate excluded from screenshot synthetic input by design, matches non-screen behavior)
• Tab completion shows /explain in suggestion list with correct icon and description