Fix emoji suggestions: show last in panel, never auto-select

[iamshakibali]

Problem

While typing, emoji candidates from the riti engine were (a) shown at the front of the candidate panel and (b) auto-selected as the default — so pressing space would commit an emoji instead of the relevant word.

Changes (Lekho/Sources/InputController.swift)

• Emoji-last ordering — the panel now shows word candidates in riti's original ranking, with emoji-only candidates appended at the end. A new isEmojiCandidate(_:) helper detects emoji-only candidates (pure emoji/symbol content); anything containing Bengali or Latin letters/digits is treated as a word candidate.
• Emoji never auto-selected — a new avoidEmojiDefault() step moves the default selection to the first word candidate whenever riti's remembered/top selection lands on an emoji. Emoji are only committed by explicit user action (arrow keys, Tab, number key, or click).
• Display ↔ engine index mapping — since the panel order differs from the engine order, selectedIndex is now a display index and is converted at every riti boundary (riti_get_suggestion_for_key previously-selected index, marked/pre-edit text, commits, remembered selection, phonetic-first default). Navigation, digit keys, mouse clicks, and commits all follow the new order consistently.

Behavior

Before	After
Emoji at top, auto-selected, committed on space	Emoji at bottom, unselected; space commits the top word
Word suggestions pushed below emoji	Words keep riti's ranking

Testing

• ✅ swiftc -typecheck passes with zero errors
• ✅ Built with make build, installed to ~/Library/Input Methods, verified live: typing boish shows বিশ et al. selected by default with 🎉 unselected at the bottom; space commits the word; emoji only via explicit selection