feat: dynamic paths — date, hostname, user and env placeholders

[KiryuuLight]

Source and destination paths can now contain placeholders, resolved on every run:

• Placeholders — {now} / {now:FORMAT} (any strftime format), {utcnow}, {hostname}, {user}, plus $VAR / ${VAR} environment variables. Expanded internally; no shell is involved, so profile fields never execute anything. {{ and $$ are literal escapes.
• A literal % in a path is untouched — the delimited {…} syntax means an existing folder like 100%done keeps its name.
• Unknown placeholders and unset variables stay literal, so a typo like {nowx} is visible in the preview instead of silently resolving to the wrong destination.
• Nested destinations now work — rsync only creates the final directory, so a path like {hostname}/{now:%Y-%m-%d} previously failed at run time with mkdir … No such file or directory even though the dry-run preview reported success. prepare_dest now creates missing parents for local sync destinations, as it already did for snapshots.

Related to #1. Remote destinations with 2+ new levels are still unsupported (parents can't be created on the far side).

[Copilot]

Pull request overview

Adds “dynamic paths” support so saved rsync profiles can include date/host/user placeholders and environment variables that are expanded internally on each run (no shell). This also improves reliability for nested local sync destinations by creating missing parent directories before invoking rsync.

Changes:

• Implement placeholder ({now}, {now:...}, {utcnow}, {hostname}, {user}) and environment variable ($VAR, ${VAR}) expansion via crate::paths::expand_path.
• Ensure nested local sync destinations work by creating destination parent directories in prepare_dest (not just snapshots).
• Add unit/integration tests plus README documentation for dynamic paths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/ui/fields.rs	Uses expand_path for status checks and updates dest hinting; adds tests for placeholder paths and nested parents.
src/rsync.rs	Switches local expansion to expand_path and extends prepare_dest to create parents for sync; adds tests including a live rsync nested-dest case.
src/paths.rs	Introduces placeholder + env expansion pipeline (expand_path) and associated tests.
README.md	Documents the new dynamic path placeholder/env syntax.

Comments suppressed due to low confidence (1)

src/paths.rs:171

• path_hits currently treats any non-absolute buffer containing ':' as remote. With formatted placeholders ({now:...}), the literal ':' in the placeholder syntax can incorrectly disable local path completion for inputs like ~/{now:%Y-%m-%d}/. Using the expanded path for the remote-path heuristic avoids this false positive.

pub fn path_hits(buffer: &str) -> Vec<String> {
    if buffer.contains('@') || (buffer.contains(':') && !buffer.starts_with('/')) {
        return Vec::new();
    }

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