TUI: load inline images lazily, after the text renders

Split out of #246, which #259 closes for the bounded, full-thread read and the image budget.

`mailView.fetchTopic` and `journalView.fetchJournalEntry` still fetch every image within the budget (`internal/tui/image_budget.go`) before the message that renders the text is delivered, so a thread whose images are slow holds the text back for up to `imageFetchDeadline`. Two steps remain from #246's scope:

- Render the text first and deliver images through follow-up messages, with the placeholders reserving their cells so the layout does not jump.
- Fetch an image only as the viewport reaches it, and cancel what the reader has scrolled past or navigated away from.

The budget stays the bound either way; what moves is when inside it each image is requested.

Start with mailView.fetchTopic and journalView.fetchJournalEntry, then read internal/tui/image_budget.go and trace how rendered messages and follow-up messages reach the TUI. Done means text renders without waiting for slow images, placeholders reserve their cells, images load as the viewport reaches them, and off-screen or abandoned requests can be canceled while the budget remains the bound.

Written by the indexing model from the issue text.