Add remaining type aliases, rewrite settings tab as syntax TL;DR

- Add age (birthday), remaining (countdown), elapsed (since) aliases,
  alongside the existing bday/until/difference.
- README: document new aliases in each type's header and the Type
  Aliases section.
- Settings tab rewritten: drop the stale GitHub doc link and hardcoded
  pre-redesign examples (countdown+from, auto-colon labels). Replace
  with a compact syntax TL;DR \u2014 all four types with their aliases,
  fields, and one-line descriptions, plus relative dates, frontmatter
  fallback, and inline/fenced examples \u2014 and link to the README on
  Gitea (was pointing at a stale GitHub URL).

Verified: aliases tested against the real shipped config parser; the
settings tab's actual display() (found via app.setting.pluginTabs) was
invoked live in Obsidian and rendered exactly the intended content with
no errors.

README.md

@@ -21,7 +21,7 @@ Use inline code for quick calculations: `date-calc: birthday=1992-08-16`
 
 ## Supported Calculation Types
 
-### Birthday (`birthday`)
+### Birthday (`birthday`, `bday`, `age`)
 
 Calculates age and time until next birthday.
 
@@ -45,7 +45,7 @@ birthday: 2000-05-21
 label: "John's age:"
 ```
 
-### Countdown (`countdown` or `until`)
+### Countdown (`countdown`, `until`, `remaining`)
 
 Time remaining until a target date, **always measured from now**. For a comparison between two fixed dates, use `diff` instead.
 
@@ -64,7 +64,7 @@ label: "New Year:"
 to: 2025-12-31 23:59
 ```
 
-### Since (`since`)
+### Since (`since`, `elapsed`)
 
 Time elapsed since an event (or time until, if the event is in the future) — always measured against now.
 
@@ -81,7 +81,7 @@ type: since
 since: 2024-12-20
 ```
 
-### Difference (`diff`)
+### Difference (`diff`, `difference`)
 
 The time difference between two fixed dates — neither has to be "now". For a countdown relative to the present moment, use `countdown` instead.
 
@@ -187,8 +187,9 @@ Settings → "Hide result while cursor inside" prevents rendered output from sho
 
 For convenience, use these shortcuts:
 
-- `birthday` = `bday`
-- `countdown` = `until`
+- `birthday` = `bday` = `age`
+- `countdown` = `until` = `remaining`
+- `since` = `elapsed`
 - `diff` = `difference`
 
 ## Custom Styling